The general principle of operation of a matrix touch surface of “projected capacitive” type consists in measuring the capacitance variations caused by the touches that are to be detected. Two scanning modes are usually used. In the so-called “self capacitance” mode, the capacitance of each row and of each column of the matrix is tested. In the “mutual capacitance” mode, all the intersections between the different rows and columns are tested. When the touch surface is of large size and it comprises a large number of rows and columns, the “mutual capacitance” mode cannot be used alone because the scanning time is too long. It is, generally, used to complement a scanning of the “self capacitance” type.
In the latter mode, the injection and the measurement of the signal can be done in different ways. One of the standard modes is to inject sinusoidal signals at particular frequencies. This method offers the advantage of minimising the radiated emissions and of allowing the simultaneous use of several so-called “orthogonal” frequencies. For the record, two frequencies are said to be orthogonal if there are two integer numbers that are not multiples of one another such that the product of the first integer number by the first frequency is equal to the product of the second integer number by the second frequency.
The measurement of the signal can be done by capacitive bridge, by injecting a current and by measuring a voltage or, conversely, by injecting a voltage and by measuring a current. FIG. 1 illustrates the latter mode of operation. This figure schematically represents a touch matrix M comprising a plurality of rows Li and of columns Cj. The addressing electronics EL of the rows and EC of the columns schematically comprise:
a digital-analogue conversion stage DAC, which injects a sinusoidal high frequency voltage denoted V onto the row or the column of the touch surface
an analogue-digital conversion stage ADC for measuring the received current.
The measurement of the current is ensured by an electronic processing chain which determines the position of the touch or touches on the touchscreen and ensures the retransmission of the processed signals externally, generally to a display device coupled with the touchscreen. Depending on the hardware resources available, measurements can be made on several rows simultaneously. In the same way, the measurements can be done at the same time on the columns, subject to the use of a second frequency F2 different from the first frequency F1 of the rows. The hardware resources necessary depend on the number of rows and of columns of the touch surface an on the desired scanning time.
When the matrix sizes increase, this method presents a certain number of drawbacks linked to the following physical parameters:
resistivity of the row and column access lines because of their small width and of the materials used;
capacitance of the rows and of the columns which increase with their dimensions;
resistivity of the actual rows and columns due to the transparent material used. In some applications, the optical performance requirements prohibit the use of materials that are less resistant but also less transparent.
Ultimately, on the matrices of large dimensions, a loss of sensitivity is observed when the touch is situated at row end. Generally, as illustrated in FIG. 2, to resolve this problem, two matrix touchscreens M1 and M2 are juxtaposed side by side to form a double-size touchscreen. By using different frequencies F1, F2 and F3, F4 from one screen to the other, any uncontrolled interaction is avoided. The main drawback with this solution is that there is a central discontinuity between the two touchscreens which is prejudicial to the good perception of the overall image.